In the oil and gas industry, there are a number of drilling, completion and production operations which require the remote operation of downhole tools, devices and equipment using commands or actions carried out at surface.
This can be achieved in numerous ways, perhaps the most common of which is the use of hydraulic pressure generated in the fluid pumped through the drilling or production tubulars to activate a downhole tool or device. In some instances, the pressure of the pumped fluid flow acting across a flow restriction such as a nozzle can be used to apply a force across a piston which in turn activates a tool downhole or device, such as an expandable underreamer, an expandable casing mill, a side port circulating tool or similar device. However, as fluid circulation is required for other operations, such as in the drilling operation itself, it is important that the activation mechanism is selective and can differentiate between the fluid flow required for the drilling process and the fluid flow required to activate the downhole tool or device. In this situation, there are again a number of approaches in current use. For example, tool activation may be achieved by application of a pressure above that required for normal drilling operations, the applied pressure acting on a spring loaded piston. In this example, the downhole tool or device will operate whenever the pressure exceeds a threshold pressure and which is sufficient to overcome the strength of the return spring. Although effective in some applications, this approach has a number of drawbacks. For example, it may not be possible for an operator to obtain a positive indication at surface that the desired activation has taken place. Also, the activation may not be a positive action but happen slowly or partially unless shear pins are employed to ensure that increased pressure is applied above that required to overcome the return spring force. Also, only partial activation may be achieved due to spring tension seal friction and or other factors such as the ingress of mud solids into the moving parts of the system. Also, after activation, the tool will de-activate due to the force of the return spring if and whenever the flow and pressure is reduced below the threshold pressure.
An alternative approach is the use of a ball which is dropped or pumped down from surface so as to land on a ball seat in the piston. The use of a ball may overcome some of the above drawbacks and may provide a relatively simple and low cost activation system. For example, there will be a positive pressure indication when the ball lands on the seat. There will also be a positive activation of the tool when the pressure on the activation piston is sufficient to shear the shear pins holding the tool in the non-activated condition. Also, the flow rate required for activation can be very low. However, these applications too have a number of drawbacks. For example, shear pins permit only a single operation and cannot be replaced until the tool or device is retrieved to surface. Also, the ball introduced at surface will take a considerable time to pump down to the tool or device before activation can take place, this time representing a significant cost to an operator. Also, when located on the seat the ball creates an undesirable restriction in the bore which may inhibit or prevent the passage of other tools. Repeat operations may also not be possible with the more basic systems.
As a further alternative to the techniques described above, more complex systems are currently being used in the industry which involve dropping a number of sequential balls or darts into the bore to land on a deformable ball seat or releasing mechanism mounted within a sprung loaded actuation piston. After actuation, the balls or darts may be released to be caught in a catcher or holder below the tool or device. These systems beneficially allow a number of repeat operations to be carried out but in some situations can be unreliable. They may also still suffer many of the disadvantages described above.
More recently, RFID (Radio Frequency Identification) tags have been used to transfer instructions to a tool or device to be activated as the RFID tag is pumped through it. The use of RFID tags permits a high number of repeat operations to be carried out and permits more sophisticated instructions to be communicated to the tool being activated, including the ability to differentiate between a number of downhole tools. The use of RFID tags may also overcome the problem of bore restriction associated with drop ball techniques. Nevertheless, this approach represents a comparatively complex and costly solution which requires sophisticated downhole electro-mechanical actuation systems to operate.